Stephen Benskin
Allowing Ads
A frequent way of answering questions on this forum is to reference either a book or website that describes a procedural system of testing. There are a plethora of photographic systems and variations on systems. They can’t all be 100% correct, yet we tend to pass them along without comment. I’ve often seen half a dozen different testing methods recommend over the course of a single thread. Mostly these systems are primarily about methodology or how to test. Few present evidence that there is any proof of their assumptions apart from the claim that it all seems to work. I don’t think this type of justification should be sufficient.
We do ourselves and those we intend on helping a disservice if we accept and pass along any claims without a healthy bit of skepticism. How much additional insight could we gain by simply doing a little critical thinking forcing us to reevaluate our assumptions about what’s behind a given testing method? What is the reasoning behind the claims? Do the facts support the claims? How accurate are the claims with the results? Even if the answers aren’t a revelation, at the very least they will strengthen our knowledge base, and we will be less likely to pass along potential misinformation.
So, here’s the challenge. Using supporting evidence and without using anecdotal evidence, circular reasoning, or arguments from authority, breakdown the two speed testing methods below, one from a website and one from a book, and prove which claims and facts are valid, which are not, and whether either will result in the type of results they claim. I’ve found that if I approach a question from a number of different directions and still get the same answer, then the answer is likely correct.
Method 1:
Making Test Exposures
1. Set up a target, white, gray or any medium color will do. It should be large enough to fill the frame from six feet away. The target should be smooth - wrinkles can cause shadows or uneven brightness.
2. Light the target evenly. Meter readings in the center and at the edges should not vary more than 1/3 of a stop.
3. Set your meter at the film’s ISO rating (e.g. 400 for Tri-x). This will be called the test speed.
4. Take a reflected meter reading of the target; choose a shutter speed that will allow you to bracket without using the slowest or fastest speeds.
5. Set your camera for manual exposure; any auto-exposure setting will make the test meaningless.
6. Record your exposures as you make them.
7. Make the following exposures
a. Meter reading - Zone V
b. Blank frame - FB+F film base plus fog
c. 5 stops less thank the meter reading. (close down the lens, you may need to increase the shutter speed don’t forget to put it back at your original setting as soon as possible) - Zone 0
d. 4 stops less than the meter reading -Zone I
e. 3 stops less than the meter reading - Zone II
f. 2 stops less than the meter reading - Zone III
8. Develop the film normally. Using the developer, time temperature you usually use.
Evaluating the test
To get the true film speed you must determine which test exposure produces Zone I. There are two ways to do this: you could make a visual determination by judging prints or you could use a densitometer. The densitometer is more accurate but you need to have access to the equipment.
The Densitometer Method
1. Take a reading of the blank frame (exposure b)
2. Take readings of all other frames; the frame with 0.10 - 0.15 more density than the blank frame (b) is the speed frame.
If test frame c is the speed frame then your true speed is 2x the test speed.
If test frame d is the speed frame then your true speed is equal to the test speed.
If test frame e is the speed frame then your true speed is ½ the test speed
If test frame f is the speed frame then your true speed is ¼ the test speed
For example: You tested Kodak Tri-x (ISO 400) and your blank frame (b) reads 0.23
Frame c reads 0.23
Frame d reads 0.26
Frame e reads 0.33
Frame f reads 0.41
Your speed frame is e; if your test speed is ISO 400 then your true speed or EI (exposure index) is ½ that or 200.
Method 2:
The set-up to the book method is similar to Method 1.
A step tablet is taped to a sheet of film and loaded into a holder. “Meter the target using the film speed recommended by the manufacturer and set the lens and shutter speed at values corresponding to a Zone X exposure – five stops more than the settings indicated by our light meter.”
Process the film and read the resulting densities using a densitometer and plot them. “Because the density gradients of the calibrated strip subtract from the intensity of light striking the film, the densities of the steps are plotted along the horizontal axis as negative numbers starting from the value of 0.0 at the far right. The average density of the portion of the negative not covered b the film strip is D-Max and corresponds to the density produced by a Zone X exposure after development. Each decrease of 0.30 units of density from the right axis is equivalent to one zone less exposure.”
Zone I is 9 stops below Zone X. If Zone X equals a step tablet density of 0.0, Zone I should fall at 0.30 * 9 or a density of 2.70 corresponding to the step tablet. If the plotted curve crosses 0.10 at -2.70 then the speed of the film equals the film speed recommended by the manufacturer. The curve crosses 0.10 at another point along the x-axis, the speed is equivalent to the speed setting plus or minus the amount it is offset. For instance, if the curve crosses 0.10 at -2.40 then the film has an EI of one stop slower than the film speed setting.
We do ourselves and those we intend on helping a disservice if we accept and pass along any claims without a healthy bit of skepticism. How much additional insight could we gain by simply doing a little critical thinking forcing us to reevaluate our assumptions about what’s behind a given testing method? What is the reasoning behind the claims? Do the facts support the claims? How accurate are the claims with the results? Even if the answers aren’t a revelation, at the very least they will strengthen our knowledge base, and we will be less likely to pass along potential misinformation.
So, here’s the challenge. Using supporting evidence and without using anecdotal evidence, circular reasoning, or arguments from authority, breakdown the two speed testing methods below, one from a website and one from a book, and prove which claims and facts are valid, which are not, and whether either will result in the type of results they claim. I’ve found that if I approach a question from a number of different directions and still get the same answer, then the answer is likely correct.
Method 1:
Making Test Exposures
1. Set up a target, white, gray or any medium color will do. It should be large enough to fill the frame from six feet away. The target should be smooth - wrinkles can cause shadows or uneven brightness.
2. Light the target evenly. Meter readings in the center and at the edges should not vary more than 1/3 of a stop.
3. Set your meter at the film’s ISO rating (e.g. 400 for Tri-x). This will be called the test speed.
4. Take a reflected meter reading of the target; choose a shutter speed that will allow you to bracket without using the slowest or fastest speeds.
5. Set your camera for manual exposure; any auto-exposure setting will make the test meaningless.
6. Record your exposures as you make them.
7. Make the following exposures
a. Meter reading - Zone V
b. Blank frame - FB+F film base plus fog
c. 5 stops less thank the meter reading. (close down the lens, you may need to increase the shutter speed don’t forget to put it back at your original setting as soon as possible) - Zone 0
d. 4 stops less than the meter reading -Zone I
e. 3 stops less than the meter reading - Zone II
f. 2 stops less than the meter reading - Zone III
8. Develop the film normally. Using the developer, time temperature you usually use.
Evaluating the test
To get the true film speed you must determine which test exposure produces Zone I. There are two ways to do this: you could make a visual determination by judging prints or you could use a densitometer. The densitometer is more accurate but you need to have access to the equipment.
The Densitometer Method
1. Take a reading of the blank frame (exposure b)
2. Take readings of all other frames; the frame with 0.10 - 0.15 more density than the blank frame (b) is the speed frame.
If test frame c is the speed frame then your true speed is 2x the test speed.
If test frame d is the speed frame then your true speed is equal to the test speed.
If test frame e is the speed frame then your true speed is ½ the test speed
If test frame f is the speed frame then your true speed is ¼ the test speed
For example: You tested Kodak Tri-x (ISO 400) and your blank frame (b) reads 0.23
Frame c reads 0.23
Frame d reads 0.26
Frame e reads 0.33
Frame f reads 0.41
Your speed frame is e; if your test speed is ISO 400 then your true speed or EI (exposure index) is ½ that or 200.
Method 2:
The set-up to the book method is similar to Method 1.
A step tablet is taped to a sheet of film and loaded into a holder. “Meter the target using the film speed recommended by the manufacturer and set the lens and shutter speed at values corresponding to a Zone X exposure – five stops more than the settings indicated by our light meter.”
Process the film and read the resulting densities using a densitometer and plot them. “Because the density gradients of the calibrated strip subtract from the intensity of light striking the film, the densities of the steps are plotted along the horizontal axis as negative numbers starting from the value of 0.0 at the far right. The average density of the portion of the negative not covered b the film strip is D-Max and corresponds to the density produced by a Zone X exposure after development. Each decrease of 0.30 units of density from the right axis is equivalent to one zone less exposure.”
Zone I is 9 stops below Zone X. If Zone X equals a step tablet density of 0.0, Zone I should fall at 0.30 * 9 or a density of 2.70 corresponding to the step tablet. If the plotted curve crosses 0.10 at -2.70 then the speed of the film equals the film speed recommended by the manufacturer. The curve crosses 0.10 at another point along the x-axis, the speed is equivalent to the speed setting plus or minus the amount it is offset. For instance, if the curve crosses 0.10 at -2.40 then the film has an EI of one stop slower than the film speed setting.